The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast with circuitry for protecting against a lamp-to-earth-ground fault condition.
Fluorescent lamps used with electronic ballasts periodically fail and require replacement. In most cases, replacement of a failed lamp is performed while AC power is still applied to the ballast; this practice is sometimes referred to as xe2x80x9clive relamping.xe2x80x9d Since many newer ballast designs have non-isolated outputs, the possibility exists for high frequency output current to travel from the ballast output, through the lamp, through the person replacing the lamp, to fixture ground. Because an electrical shock may be suffered under such circumstances, safety agencies such as Underwriters Laboratories now require that ballasts be tested for this condition. Thus, standards have been established for the maximum current that is allowed to flow from the ballast output through the lamp to fixture ground. For many ballasts, these standards are readily met. However, for some ballasts, such as those models which are designed to operate with higher line voltages (e.g., 277 volts) or shorter lamp lengths (e.g., 2 foot lamps), these standards can be met only by incorporating special protective circuitry in the ballast.
Some ballast manufacturers have attempted to address the problem of excessive lamp-to-earth-ground current by trying to sense the high frequency leakage current that, in the event of a fault condition, flows out of the ballast output, into the grounded fixture, and back into the ballast via the ballast ground wire that is electrically connected to the fixture during ballast installation. An example of such an approach is described in U.S. Pat. No. 5,363,018. The main problem with this type of detection circuit is that this same type of leakage current normally flows even in the absence of a fault condition, and is actually quite desirable because it aids lamp ignition. Moreover, because the voltage applied to the lamps prior to ignition is much higher than voltage applied after ignition, the magnitude of this xe2x80x9cnormalxe2x80x9d leakage current will be many times higher during the start-up mode than during the steady-state operating mode. Because the magnitude of the normal leakage current that flows into the ballast ground during normal starting conditions can be very close to the magnitude of the undesirable leakage current that flows through the body of a person who accidentally touches the ballast output and fixture ground, the prior art circuits cannot accurately discriminate between xe2x80x9cnormalxe2x80x9d leakage current and the leakage current that occurs due to a true fault condition.
What is needed, therefore, is a ballast with a protection circuit that is capable of more reliably detecting a lamp-to-earth-ground fault condition. A ballast with such a protection circuit would represent a significant advance over the prior art.